A pneumatic braking system of a trailer vehicle is typically connected via two pneumatic lines to a drawing vehicle. A line conducts control pressure via a control connection to the trailer vehicle. The other line conducts reservoir pressure via a supply connection to the trailer vehicle. As a result of the legal provisions in Germany and other countries, the control connection has a yellow coupling head and the supply connection has a red coupling head.
A safety function is integrated in the event that the line for the reservoir pressure severs. To this end, the pressure in the line is monitored. In the case of an excessive drop in pressure, the trailer vehicle is automatically braked by accumulator spring brakes.
The trailer vehicle has a reservoir for compressed air which is fed via the supply connection. As a result, the trailer vehicle can be braked several times in quick succession without the compressed air required for this having to be supplied simultaneously via the supply connection.
Modern pneumatic braking systems have combined brake cylinders, namely with diaphragm service brake cylinder and spring accumulator brake cylinder in a joint assembly. This has the advantage that brake pistons, brake levers and brake discs or brake drums only have to be present once for each assembly. Only spring accumulator brake cylinders and service brake cylinders are present next to one another for each assembly. Such combined brake cylinders are also referred to as tristop cylinders.
In order to avoid mechanical overloading, spring accumulator brake cylinders and service brake cylinders should not simultaneously exert braking force. The forces of spring accumulators thus act within the brakes after a drop in pressure on the supply connection or in the subsequent lines. If the service brakes are now additionally activated by a driver because sufficient pressure is still present in the reservoir, the forces resulting from the actuation of the service brake cylinders additionally act within the brakes. An addition of forces arises which can bring about damage or even destruction of components.
Additive actuation of spring accumulator brakes and service brakes can also occur during normal parking of the trailer vehicle. The spring accumulator brakes are simultaneously parking brakes. If the parking brakes are activated and the service brakes are additionally actuated by the driver, the addition of forces discussed above also occurs.
An overload protection is required to avoid the stated damage or destruction.
A valve arrangement for the automatic braking of a trailer vehicle via the spring accumulator brakes in the event of severing of the reservoir line is disclosed in EP 2 190 706 B1. Aeration of the spring accumulator brake cylinders is carried out either by the pressure in the reservoir or by service brake pressure. Switching between the two stated pressures is carried out via a control input. In the normal case, the control input conducts pressure from a line following on from the supply connection.